The mutagenic and carcinogenic effects of electrophiles such as N-methyl-N-nitrosourea are mainly due to the O6-alkylation of guanine in DNA. To protect themselves against DNA-alkylation, mammals and bacteria possess a protein, O6-alkylguanine-DNA alkyltransferase (AGT) which repairs these lesions. AGT transfers the alkyl group from the position O-6 of alkylated guanine and guanine derivatives to the mercapto group of one of its own cysteines, resulting in an irreversibly alkylated AGT. The underlying mechanism is a nucleophilic reaction of the SN2 type which explains why not only methyl groups, but also benzylic groups are easily transferred. As overexpression of AGT in tumour cells is the main reason for resistance to alkylating drugs such as procarbazine, dacarbazine, temozolomide and bis-2-chloroethyl-N-nitrosourea, inhibitors of AGT have been proposed for use as sensitisers in chemotherapy (Pegg et al., Prog Nucleic Acid Res Mol Biol 51: 167-223, 1995). U.S. Pat. No. 5,691,307 describes O6-benzylguanines carrying various substituents in the benzyl group, and their use for depleting AGT levels in tumor cells and thereby increasing responsiveness to alkylating anti-tumor drugs. Likewise, WO 97/20843 discloses further AGT depleting compounds representing O6-benzyl- and O6-heteroarylmethyl-pyrimidine derivatives.
DE 199 03 895 discloses an assay for measuring levels of AGT which relies on the reaction between biotinylated O6-alkylguanine derivatives and AGT which leads to biotinylation of the AGT. This in turn allows the separation of the AGT on a streptavidin coated plate and its detection, e.g. in an ELISA assay. The assay is suggested for monitoring the level of AGT in tumour issue and for use in screening for AGT inhibitors.
WO 01/85221 proposes the use of radiolabelled fluoro- or iodo-substituted O6-benzyl-guanines for detection of AGT and monitoring the level of AGT.
Damoiseaux et al., ChemBiochem. 4: 285-287, 2001, disclose modified O6-alkylated guanine derivatives incorporated into oligodeoxyribonucleotides for use as chemical probes for labelling AGT, again to facilitate detecting the levels of this enzyme in cancer cells to aid in research and in chemotherapy.
PCT/GB02/01636 (WO 02/083937) discloses a method for detecting and/or manipulating a protein of interest wherein the protein is fused to AGT and the AGT fusion protein contacted with an AGT substrate carrying a label, and the AGT fusion protein detected and optionally further manipulated using the label. Several AGT fusion proteins to be used, general structural principles of the AGT substrate and a broad variety of labels and methods to detect the label useful in the method are described.